One way in which both of Tim O'Brien's Vietnam War novels Going after Cacciato and The Things They Carried relate concerns the theme of reality vs. imagination. In both novels, the soldiers develop their own sense of reality as a survival mechanism.
The theme of reality vs. imagination is most evident in Going after Cacciato. The novel is divided up into three sections. In the middle section, titled "Observation Post," primary character Paul Berlin is standing watch during the night at his observation post. As he does so, Paul relays his thoughts. One of his primary thoughts is imagining the squad hunting down Cacciato, who has gone away without leave (AWOL); he goes so far with his fantasy as to imagine the squad pursuing Cacciato all the way into Paris, which, of course, is very unrealistic. But as Paul continues to daydream about the hunt, his mind also flashes back to true events he has lived through over the past half year. Among the traumatic events he has lived through include a soldier named Billy Boy Watkins dying "of fright, scared to death" on the battle field; soldier Frenchie Tucker dying of being "shot through the nose"; soldier Bernie Lynn dying while trying to dig a tunnel; and Lieutenant Sidney Martin being killed by his subordinates for forcing them to dig the tunnel. Paul relays the story of each death multiple times, and each time he goes into the story, he goes into more and more horrific details. At a point, the gruesome realities Paul has lived through become so horrific that they seem fantastic, they seem more made-up than his daydreams about pursuing Cacciato into Paris.
O'Brien's point is to assert that there is no clear dividing line between fantastically horrific events of the Vietnam war and reality. The reality that the soldiers of the war had to experience was so horrific that those civilians who experienced a different reality will never be able to fully grasp the Vietnam Vet's reality. What's more the soldiers needed to indulge in the fantastical in order to survive because doing so offered an escape from their horrific reality.
Similarly, O'Brien shows that there is no clear distinction between fantasy and reality in The Things They Carried. O'Brien's main motif is a description of the various items the soldiers carried with them during the war. Some of those items are related to the men's reality; some of those are actually related to the men's fantasies. The items they carried pertaining to reality included their gear such as rucksacks, "can openers, pocket knives, heat tabs, wristwatches, dog tags," etc. (p. 2). First Lieutenant Jimmy cross is an example of a character who carries items associated with fantasy rather than reality. He carries two pictures of a girl he knows named Martha and all of her letters. He knows her letters aren't love letters, but he wishes they were and is hoping to be able to marry her when he returns. Fantasizing that he and Martha have a deeper relationship than they really do helps to save his mind from dwelling on the reality he is suffering while in the war. Anything the men carry that is associated with fantasy is their saving grace--it helps to rescue them from their present reality by serving as a needed distraction.
Hence, just like Paul in Going After Caccio, men in The Things They Carried indulge in fantasy as a means of survival, especially survival from a reality that's so horrific it seems fantastical.